This invention relates to speed reducers, and in particular, to a method of assembling 90.degree. speed reducers.
Speed reducer assemblies are widely used for applications in which a rotational speed, such as an output from a motor, is reduced to perform an intended function. Speed reducers commonly include a housing which supports a worm and a worm gear in meshing contact. In a 90.degree. reducer, the axis of the worm gear output shaft is perpendicular to the worm.
To increase the useful life of a reducer assembly, it is important for the center of the worm to be aligned as closely as possible to the plane of the center of curvature of the worm gear. If the worm is offset from the center of curvature of the worm gear to any appreciable degree, the worm will prematurely wear down the worm gear, reducing the life of speed reducer assembly. Because the center of curvature of the worm gear does not necessarily coincide with the center of the worm gear plate, properly positioning the gear in the housing is difficult.
The majority of 90.degree. speed reducers presently used have a housing which supports the worm through side walls. The front and back of the housing define openings which are closed by brackets which support bearings of the worm gear assembly. To appropriately position the worm gear in these assemblies requires a hit and miss method. This method includes placing shims of an anticipated depth between the housing and bracket and then securing the bracket to the back wall of worm gear assembly bearing. The position of the worm gear in relation to the worm is then checked. If the position is not right, the assembly is taken apart, shims are removed or added, as needed, and the assembly is put back together. This procedure is repeated until the worm gear is properly positioned with respect to the worm.
Once the worm gear is properly positioned, an anticipated number of shims are placed in the front bracket and the front bracket is secured to the assembly to close it. The output shaft, which extends out the front of the assembly will have some axial movement. This axial movement must be within certain tolerances. The movement, or end play, must then be determined. If it is outside of the tolerances, the bracket must be removed, shims removed or added as needed, and then the bracket is reattached. The end play of the output shaft is then checked, and if not within tolerances, the process is repeated until the end play is within tolerances. As can be imagined, this process can require a substantial amount of labor, increasing the cost of the assembly. Further, the measurements are prone to human error, and two people may put together the same assembly with different shim depths.
The shims that are used in the above described assembly acre actually made of a plurality of shim pieces of a predetermined thickness. The appropriate sized shims are placed together in the assembly to arrive at the proper shim thickness. These shims have an outer periphery that is co-extensive with the periphery of the front and back brackets. The bolts which secure the brackets to the housing therefore pass through the shims, necessitating total removal of the bracket when the shim depth must be changed during assembly of speed reducers.
A housing having a closed front and open back (i.e., having one removable bracket) has been produced. This assembly, however, only allows for the adjustment of shims on the removable bracket. The worm gear assembly includes the shaft, worm gear, spacers, and bearing assemblies all of which have tolerances. These tolerances are stacked when the worm gear assembly is put together. This speed reducer assembly, unlike the above described assembly, does not allow for compensation of these tolerances. Therefore it is nearly impossible to properly align the worm and worm gear of this assembly.
Speed reducer assemblies have been provided which attempt to aid in the proper alignment of the worm and worm gear of the speed reducer. U.S. Pat. No. 4,261,218, to Eagan, Sr., discloses a speed reducer assembly which has a threaded adjuster, which when rotated, axially moves the input shaft. This will move the worm relative to the worm gear to minimize or eliminate back lash between the worm and worm gear. However, it will not eliminate the trial and error inherent the assembly of 90.degree. worm gear speed reducers.